Context. We report on the detection of an atomic jet associated with the protostellar source SMM1 (FIRS1) in Serpens. The morphological and physical characteristics of the atomic jet suggest that there is a more evolved protostellar companion to the Class 0 source SMM1.

Aims. We unravel the molecular and atomic emission around the protostellar source Serpens-SMM1, identify the emission line origin, and assess the evolutionary stage of the driving sources.

Methods. The surroundings of SMM1 were mapped with the Spitzer Infrared Spectrograph (IRS) in slit-scan mode. The complex outflow morphology of the molecular (H2) and atomic ([FeII], [NeII], [SiII], [SI]) emission from Spitzer is examined along with deconvolved Spitzer IRAC and MIPS images and high-velocity CO J = 3–2 outflow maps. The physical conditions of the atomic jet are assessed assuming LTE, non-LTE conditions, and shock models.

Results. The atomic jet is firmly detected in five different [FeII] and [NeII] lines, with possible contributions from [SI] and [SiII]. It is traced very close to SMM1 and peaks at ~5″ from the source at a position angle of ~125°. H2 emission becomes prominent at distances > 5″ from SMM1 and extends at a position angle of 160°. The morphological differences suggest that the atomic emission arises from a companion source, lying in the foreground of the envelope surrounding the embedded protostar SMM1. The molecular and atomic emissions disentangle the large-scale CO emission into two distinct bipolar outflows, giving further support to a protobinary source. The LTE and non-LTE analysis at the peaks of the [FeII] jet show that emission arises from warm and dense gas (T ~ 1000 K, ne ~ 105–106 cm-3). These conditions are suggestive of dissociative J-type shocks, and this is further supported by strong water-maser emission observed on the axis of the atomic jet. The mass flux of the jet derived independently for the [FeII] and [NeII] emission is ~107 M⊙ yr-1, pointing to a more evolved Class I/II protostar as the driving source. Comparisons of the large-scale outflow and atomic jet momentum fluxes show that the latter has adequate thrust to support the CO outflow.

Conclusions. The atomic jet detected by Spitzer for the first time gives the opportunity to disentangle the complex outflow morphology around SMM1 into two precessing outflows. The morphological and physical properties of the outflows reveal that SMM1 is a protobinary source. The momentum flux of the atomic jet indicates that the companion to the deeply embedded Class 0 protostar SMM1 is a more evolved Class I/II source.